The mineral industry is a large consumer of chemicals. These chemicals are used in the various steps of transformation/modification/processing to which the mineral matter is subject. Thus, in the case of calcium carbonate of natural or synthetic origin, many operations known as “grinding” operations (reduction of the granulometric size of the particles) when dry or in a wet medium, or “dispersion” operations (suspension of the particles in a liquid), are undertaken.
Both these operations are made more easy through the use, respectively, of grinding aid agents, the role of which is to facilitate the mechanical action of attrition and fragmentation of the particles, and of dispersing agents, the function of which consists in maintaining the viscosity of the suspension within acceptable ranges as the mineral matter is introduced into it. The present invention relates to the methods of grinding which use grinding aid agents.
The prior art is particularly rich on the subject of such additives. For many years, it has been known that water-soluble homopolymers of acrylic acid constitute efficient agents to aid dispersion or grinding in an aqueous medium of calcium carbonate. Reference may usefully be made to the documents FR 2 539 137, FR 2 683 536, FR 2 683 537, FR 2 683 538, FR 2 683 539 and FR 2 802 830.
For the same type of application, it is also advantageous to copolymerise acrylic acid with another carboxylic monomer such as, itaconic acid, methacrylic acid or sulfonic acid, such as acrylamido-2-methyl-2-propane sulfonic acid or maleic anhydride, and/or with another monomer with ethylenic unsaturation, but without a carboxylic group, such as an acrylic ester: these variants are also described in the previous documents.
It is also known that regulating the polymolecularity index of the abovementioned polymers enables some of their performance characteristics to be optimised. This is described in the documents “Synthesis and Characterization of Poly(acrylic acid) Produced by RAFT Polymerization. Application as a Very Efficient Dispersant of CaCO3, Kaolin, and TiO2” (Macromolecules, 36(9), 3066-3077, 2003) and “Dispersion of calcite by poly(sodium acrylate) prepared by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization” (Polymer (2005), 46(19), 8565-8572). This polymolecularity index can notably be ascertained using polymerisation techniques known as “live” techniques, as illustrated in documents WO 02/070571 and WO 2005/095466. Patent FR 2 514 746, for its part, describes a method called “fractionation” which enables the polymolecularity index to be regulated by selecting polymeric chains which are longer than a given length.
It is also known that the choice of the molecular weight of these polymers may, in the case of particular methods of manufacture of calcium carbonate, improve the efficiency of the said method: patent EP 1 248 821, for example, emphasises carboxylic polymers of high molecular weight, to disperse in water a substantial quantity of a calcium carbonate derived from a step of low-concentration grinding with no polymer present.
Independently of these various strategies for improvement of the application properties of a dispersing or grinding agent with an acrylic acid base (choice of a comonomer, of a polymerisation technique, regulation of molecular weight), it is known that the particular choice of these neutralisation agents leads to appreciably improved application properties.
Thus, document EP 0 100 948 demonstrates the advantage of neutralisation using sodium and calcium ions. A generation of subsequent patents (FR 2 683 538 and FR 2 683 539), for its part, favours the magnesium/sodium ion pair. Finally, a latest generation of patents is known (EP 1 347 834 and EP 1 347 835), which is based on a partial neutralisation (not all the carboxylic sites are neutralised) using the joint action of a monovalent agent (preferentially sodium), and at least one divalent agent (preferentially calcium or magnesium).